It is known to use so-called arrayed waveguide gratings (AWGs) for multiplexing or demultiplexing optical signals of different wavelength. An AWG generally comprises two free-beam regions that are interconnected by a phase-shifting arrangement. Following the free-beam regions are waveguides for coupling light in and out. A corresponding arrangement is described, for example, in WOA96/00915 and WOA99/52003.
The post deadline papers entitled “Automatic Colorless Tunable Dispersion Compensator with Integrated Tunable Noise Filter” by C. R. Doerr et al., PD1.9, ECOC 2002 and “Multichannel Integrated Tunable Dispersion Compensator Employing a Thermo-optic Lens” by C. R. Doerr et al., FA6-1, OFC 2002, describe a structure that comprises two cascaded arrayed waveguide gratings. The symmetrical structure comprises two integrated optical phase gratings of very high order. The first phase grating separates the wavelengths of the spectrum of an optical data signal spatially. With the aid of a settable lens, it is possible to undertake phase control which is a function of location and/or wavelength and effects control of the dispersion. The second phase grating recombines the individual spectral, and now phase-shifted components and directs the optical signal to the output waveguide.
The known solution is designed in an integrated optical fashion using the technology of SiO2 on Si. The settable lens is implemented by means of the thermo-optic effect in glass. Applied for this purpose to the wafer with the optical structure, at the site at which the lens is to be located are metal heating strips that heat up when flowed through by current and thereby change the refractive index of the glass at this site.
A disadvantage of the known structure resides in that setting a specific dispersion value always requires the application of a certain electric power in order to control the lens. Since the thermo-optic effect in glass is relatively small (dn/dT≈1·10−5 1/° K.), the required temperatures, and thus also the electric power are relatively high. Again, the heating operation per se is disadvantageous, since mechanical stresses are produced in this case in the material and have a disadvantageous effect on the reliability of the structure. A further disadvantage consists in that the known structure is designed for use in 40 Gb/s systems. In the case of application to more powerful systems, for example 160 Gb/s systems, the required phase differences that have to be provided by the lens become so large that they can no longer be implemented in a practical way in glass.
The publication “40-Gb/s WDM Transmission with Virtually Imaged Phased Array (VIPA) Variable Dispersion Compensators”, Hiroki Ooi et al., Journal of Lightwave Technology, vol. 20, No. 12, December 2002 discloses a component in the case of which the spectral components of the optical signal are likewise spatially separated by a virtual phase grating. This separation is performed in such a way that the spectral components of the signal are imaged in the image plane of the structure on a straight line. A mirror with a curved surface is brought into this image plane. This curvature has the effect that a phase shift which is quadratic over the location and/or wavelength is impressed during reflection on the spatially resolved or wavelength-resolved optical signal. The phase shift which is quadratic over the wavelength effects a dispersion of the signal when it leaves the component. The three-dimensionally curved surface of the mirror also has the effect that the curvature of the phase shift which is quadratic over the wavelength, and thus the dispersion of the component change when the mirror is displaced. The dispersion of the component is therefore a function of the position of the mirror.
It is disadvantageous that this known solution is not implemented in an optically integrated fashion. It requires a relatively complicated design with a multiplicity of individual parts. In particular, the mirror with the three-dimensional curved surface constitutes an element that is difficult to produce Furthermore, a special stepping motor developed in a dedicated fashion is disadvantageously required.